Acoustic Thermometry of Ocean Climate (ATOC)
(Under Construction)

This is the site of the new ATOC page, which is under construction. ATOC as a program
ended several years ago. Acoustic thermometry in the North Pacific is supported through
the NPAL
Program. This page is to document ATOC.

Two precursors to ATOC were the Heard Island Feasibility Test (HIFT) and the Acoustic
Engineering Test (AET). The links at right go no where, however.

Background on Acoustic Thermometry

Two sound sources were installed for the first phase of the ATOC feasibility study, one on Pioneer
Seamount off central California and one north of Kauai (Figure 1-1, Location of the ATOC Sound Sources
and Receivers). The Pioneer source began transmitting in late 1995 and continued transmissions in accordance
with MMRP protocols until it was turned off at the end of 1998. The Kauai source began transmitting in
late 1997 and continued transmissions until October 1999, again in accordance with MMRP protocols. The
signals transmitted by the sources were received on SOSUS receiving arrays in the North Pacific and, for
part of the time, on a vertical receiving array located at Ocean Weather Station Papa (50°N, 145°W). The
transmissions from the Pioneer Seamount source were also recorded at various times on vertical receiving
arrays located near the Big Island of Hawaii and near Kiritimati (Christmas) Island. A small number of the
Pioneer Seamount transmissions were recorded by a receiver off New Zealand. The signals from the
Kauai source were also recorded by Russian scientists at a permanent, bottom-mounted receiver located
off Kamchatka. The primary objectives of the first phase of the ATOC feasibility study were to determine
(i) the precision with which acoustic methods could be used to measure large-scale changes in ocean temperature
and heat content and (ii) the effects, if any, which the acoustic transmissions would have on marine mammals
and other marine life. The longer-range goals of ATOC were to use acoustic thermometry data to study
seasonal and interannual temperature variability associated with a variety of oceanographic phenomena, such
as El Niño/La Niña and the PDO, and to test and improve computer models of ocean circulation. The ultimate
goal was to test and refine climate models in order to gain a better understanding of the Earth's changing
climate, including the link between global warming and sea level rise. The basic idea of acoustic thermometry is
simple. Sound travels faster in warm water than in cold water. The travel time of a sound signal from a
sound source near Hawaii to a receiver near California, for example, will decrease if the intervening ocean
warms up, and will increase if the ocean cools down. Acoustic thermometry is feasible because:
· The ocean
is nearly transparent to LF sound, so that relatively weak acoustic signals can be detected over distances of
many thousands of kilometers using appropriate signal processing techniques; and
· The speed at which sound
travels in the ocean depends primarily on temperature. (Sound speed also increases with an increase in
salinity, but in the open ocean deep water, salinity normally has only a small effect on the speed (Urick, 1983).)

Acoustic thermometry takes advantage of an acoustic waveguide deep within the ocean that traps and carries
sounds over long distances. This waveguide, known as the "sound channel" or SOund Frequency And Ranging
(SOFAR) channel, is centered on the ocean depth where the speed of sound is at a minimum. Above the
sound channel axis, sound travels faster because the water is warmer; below, sound travels faster because
the pressures are greater. Acoustic energy within the sound channel that would otherwise spread outward
to higher or lower depths is refracted (bent) back toward the sound channel axis by this difference in
speeds. The net effect is that the sound channel serves as a waveguide that transmits underwater sounds
efficiently over long distances. The sound speed minimum varies in depth based upon the temperature profile
at a given location. Since surface temperatures tend to decrease toward the poles, the sound channel axis
generally is deepest in tropical waters and shallowest in Arctic waters. Typical depths of the sound channel
in the Gulf of Alaska, for example, are 100-200 m (330-660 ft), but in warmer areas it is much deeper, on the
order of 750-1000 m (2460-3280 ft). On the north shore of Kauai, the sound channel axis is nominally at 800 m
(2625 ft), approximately at the depth of the Kauai sound source. Not all of the acoustic energy travels straight
down the axis of the sound channel. Instead, some of the sound waves cycle up close to the ocean surface, where
they are bent back down, cross the axis of the channel, and reach close to the ocean floor before being
bent back once again toward the surface. By measuring the difference in travel time between sound that
traveled a straight course down the axis of the sound channel and that which cycled in waves through various
depths of the ocean, scientists can measure how ocean temperatures vary with depth. Acoustic travel times
provide direct 3D measurements of the horizontally and vertically averaged temperature along the paths
traversed by the sound, suppressing the effects of small-scale ocean variability that dominate measurements
at a point. The great advantage of acoustic thermometry compared to other types of temperature
measurements is that such averages are just what are needed to study large-scale ocean variability
and long-term trends in ocean temperature. The information obtained is similar to that which would be
obtained for the atmosphere by averaging data from many separate weather stations. In addition,
mathematical techniques referred to as inverse methods are used to infer the horizontal and/or vertical
structure of the temperature field by combining travel time data from acoustic signals that have traveled
along different paths through the ocean. Information on the structure of the ocean temperature field
is needed to understand, for example, how the atmosphere and ocean interact to determine our weather and
climate and to study the effects of environmental variability on marine life.

Thermometry Results

Analyses of data from the ATOC project demonstrated that acoustic thermometry is a powerful tool for
making routine measurements of large-scale ocean temperature variability and heat content, as originally
hypothesized. The key results obtained to date are:
(i) Acoustic travel times can be measured with a precision of about 20-30 milliseconds (msec) at 3000-5000 km
(1620-2700 nm) ranges. For comparison, the total travel time for an underwater acoustic signal over
5000 km (2700 nm) is nearly an hour. ATOC data measurements proved to be more precise than originally
thought possible. The initial concern that acoustic scattering from small-scale ocean structure, such
as internal waves, might make accurate measurements of acoustic travel times impossible at 3000Ð5000 km
(1620-2700 nm) ranges proved to be unfounded. Transmissions over these long ranges are needed to
measure ocean gyre-scale variability, which is the scale on which ocean climate fluctuations are
expected to occur. An ocean gyre is a large, ocean-basin size (on the order of a few thousand
kilometers or nautical miles), roughly circular motion of surface water in response to wind forcing. The
travel times can then be used to estimate the range- and depth- averaged temperature with a precision of
about 0.006 °C (0.01°F) at ranges of 3,000-5,000 km (1620-2700 nm) (Dushaw, 1999; Worcester et al., 1999).
(ii) Range- and depth-averaged temperature estimates made from the acoustic travel-time data are consistent
with direct temperature measurements made with instruments lowered from ships (Worcester et al., 1999).
(iii) The observed travel time changes can be clearly related to known ocean processes. The ocean tides
are well known from other measurements, and their effect on the acoustic travel times can be predicted,
providing what is essentially a large-scale test signal. The measured and predicted travel time
fluctuations at tidal frequencies are in excellent agreement out to 5000-km (2700-nm) range. One
of the significant sources of LF sound transmission variability related to ocean temperature is seasonal
change, with the upper ocean warming during summer and cooling during winter. The ATOC data
show corresponding seasonal changes in travel times, as expected, particularly for acoustic paths that
travel north of the Subarctic Front, where the seasonal temperature changes extend to significant
depths, rather than being confined to a shallow seasonal thermocline (Dushaw et al., 1999).
(iv) The range and depth-averaged temperatures derived from ATOC are consistent with and complementary
to related estimates derived from measurements of sea-surface height. The acoustic thermometry data
from the Pioneer Seamount source have been used in conjunction with measurements of sea-surface
height made by the TOPEX/POSEIDON satellite altimeter to test and constrain a computer model of the
ocean circulation in the North Pacific (ATOC Consortium, 1998). Sea-surface height is related to
ocean temperature because of thermal expansion. It was found that previous interpretations of
sea-surface height variability as being primarily due to ocean temperature changes are
inaccurate. The effects on sea-surface height of varying ocean salinity and ocean currents also
appear to be significant. This result is important because it affects the way in which sea-surface
height data are used to test and constrain ocean circulation models. This result is also important
because it means that satellite altimetry data and acoustic thermometry data are complementary,
providing independent information on ocean structure. The altimeter has excellent horizontal
but poor vertical resolution, and the acoustic data provide information from the ocean interior
with moderate vertical resolution but poor horizontal resolution. Both have good temporal
(i.e., time-related) resolution. Consistent results for the seasonal heat storage in the ocean
are found when the acoustic and altimetry data are combined with a computer model of the ocean
general circulation. The two data types are both found to be important in constraining
the model, with the combination providing more information than either data type alone.

Marine Mammal Research Program Results

The California and Hawaii ATOC MMRPs were designed to determine the potential effects of the
acoustic transmissions on marine mammals and other marine life. They consisted of multiple components,
including:

Aerial surveys designed to determine any changes in the abundance and distribution of
marine mammals in the vicinity of the Pioneer Seamount source;

Elephant seal tagging studies designed
to determine any changes in elephant seal migratory or diving behavior in response to the Pioneer
Seamount source transmissions;

Playback studies to humpback whales off the Kona-Kohala coast of Hawaii
designed to look for behavioral changes in response to ATOC-like sounds prior to the actual ATOC source
transmissions north of Kauai;

Aerial surveys designed to determine any changes in the abundance and
distribution of humpback whales north of Kauai when the ATOC source was transmitting compared to measurements
made in previous years when the source was not transmitting;

Visual observations of humpback whale
abundance, distribution, and behavior north of Kauai to determine if there were any changes in response
to the ATOC transmissions;

Undersea acoustic recordings made with seafloor data recorders north of
Kauai to determine any changes in humpback vocalizations in response to the ATOC transmissions;

Auditory measurements on small toothed whales (odontocetes) to determine their sensitivity to the
frequencies transmitted by the ATOC sources; and

Playback studies to fish at the Bodega Bay Marine
Laboratory designed to look for behavioral changes in response to ATOC-like sounds.

Abundance and distribution. During
the MMRPs conducted in both California and Hawaii, there were no observations of overt
or obvious short-term changes in the abundance and distribution of marine mammals in response to the
transmissions of the ATOC sound sources. No species were observed to vacate the area around the sound
sources during transmissions. Intensive statistical analyses of aerial survey data showed some subtle
shifts in the distribution of humpback (and possibly sperm) whales away from the Pioneer
Seamount source during transmission periods. No statistically significant shifts in distribution
were found for any other species of marine mammal. Visual observation data from the Kauai MMRP
showed a similar small shift in mean distance of humpback whales away from the Kauai source during
transmission periods.

Behavioral measures. During the MMRPs conducted
in both California and Hawaii, there were no
observations of overt or obvious short-term changes in the behavior of humpback whales in response
to the playback of ATOC-like sounds, nor elephant seals or humpback whales in response to
to transmissions of the ATOC sound sources. Intensive statistical analyses revealed some subtle
changes in the behavior of humpback whales in response to the playback of ATOC- like sounds and
to the transmissions of the ATOC Kauai source (Frankel and Clark, 1998; Frankel and Clark, 2000). The
study results showed that the distance and time between successive whale surfacings (segment length
and segment duration) increased slightly with increasing sound levels. This result is not what
would be predicted, in that if the animals were stressed by the sound source, it might be expected
that they would remain at the surface longer because of the lower received levels there. Longer dive
durations would correspond to increased exposure to the sound source. No statistically
significant changes were found in any other behaviors measured.

Vocalizations. The Hawaii MMRP
did not find any overt or obvious short-term changes in the singing behavior of humpback whales in
the vicinity of the sound source north of Kauai. No statistically significant changes in the
underwater sound output from humpback whales in one of the frequency bands in which they vocalize was
found in the vicinity of the Kauai source.

Audiograms. The hearing sensitivity of two species
of dolphins to the ATOC sound was measured behaviorally (Au et al., 1997).
Audiograms showed that
their hearing is poor at the frequencies transmitted by the ATOC sources. The animals would have to
be extremely close to an ATOC source simply to be able to detect the transmissions.

Fish. Preliminary
playback studies of ATOC-like sounds to fish found no statistically significant responses (Klimley
and Beavers, 1998). All of the effects detected by the MMRPs were subtle and found only after intensive
statistical analyses. Bioacoustic experts concluded that these subtle effects would not adversely impact
the survival of an individual whale or the status of the North Pacific humpback whale population
(Frankel and Clark, 2000).